Unwinding apparatus for webs of material, in particular plastic films

ABSTRACT

An unwinding apparatus for webs of material, in particular plastics films (5), comprising an unwinding shaft (2) which is driven by a motor generator (4), a direction-changing guide means (6), the interior of which is under an increased pressure and feeds an air cushion, and a driven direction-changing roller (9). The technical problem lies in suppressing pulsation phenomena when unwinding sensitive plastics films. The angle over which the web of material loops around the direction-changing guide means (6) is greater than 140°. The air pressure in the interior (17) of the direction-changing guide means is at least 3 times the pressure in the air cushion (20). A pressure measuring means (11) is provided for measuring the pressure in the air cushion (20), the output signal of the measuring means being used to regulate the motor generator (4).

DESCRIPTION Technical Field of the Invention

The invention relates to an unwinding apparatus for webs of material, inparticular plastics films, having an unwinding shaft which is driven bya motor-generator, a direction-changing guide means, the interior ofwhich is under an increased pressure and feeds an air cushion, and adriven direction-changing roller.

State of the Art

Plastics films are wound up in a more or less hot condition in amanufacturing plant, following a stretching section or the like.However, after the winding operation, ageing phenomena, shrinkagephenomena, crystallisation phenomena and the like still occur, andresult in the length of the film being reduced. So that the wound coilof plastics film does not suffer due to those phenomena, which ingeneral have the effect of causing shrinkage of the film, the film mustbe wound in a very loose winding so as to permit shrinkage of theplastics film. When loose rolls of this kind are stored, they sufferfrom the formation of out-of-roundness and other deformation phenomena.The winding becomes non-round and hangs down in what may be referred toas a belly below the coil.

When a coil which has been deformed in this manner is unwound in aprocessing installation, so-called pulsation phenomena occur, whichresult in abrupt changes in the tension of the web or the winding run.Such pulsation phenomena make the subsequent processing operation moredifficult, in particular the operation of longitudinally cutting the weband the following operation of winding it up. In particular, the speedof operating on the web is detrimentally affected by the pulsationeffects.

Attempts to overcome such pulsation phenomena by adapting the speed ofrotation of the unwinding operation thereto have been found to beuseless because, with the high mass of the coil to be unwound, it istechnically not possible for the speed of unwinding rotation to beadaptively adjusted on a short-period basis. In this respect, accountshould be taken of the fact that the width of a coil may be severalmeters.

U.S. Pat. No. 2,945,637 discloses a direction-changing guide meanswherein the web of material floats on an air cushion. This arrangementseeks to provide virtually inertia-less compensation of pulsationphenomena in a strip tension regulating means of a paper processingmachine, wherein the motor generator which is connected to the unwindingroll is regulated by the increased pressure in the interior of thedirection-changing guide means, which pressure produces the air cushion.The web of material is turned through about 90° and guided around anon-driven guide roller.

Tests have shown that, when unwinding plastics films, it is notsufficient for the films to be passed around the guide means over anangle of only 90° in order totally to compensate for possibleout-of-roundness in the coil being unwound. It has also been found thata guide roller which is undriven and around which the film only passesover an angle of 90° is not sufficient for isolating the residualpulsation effect from the subsequent processing section of the plant.

Controlling the unwinding operation using the increased pressure forfeeding the air cushion, as described in U.S. Pat. No. 2,945,637 hasalso not proved to be a success in regard to plastics films. The reasonfor this is that, in order to produce a uniform air cushion pressure,there must be a high pressure loss at the air outlet openings betweenthe direction-changing guide means and the air cushion, so that the feedpressure is not representative in respect of the pressure in the aircushion.

SUMMARY OF THE INVENTION

The object of the present invention is to design an unwinding apparatussuch that pulsation phenomena are completely eliminated even whenunwinding sensitive or delicate plastics films from a loose roll.

This object is achieved by the combination of the following features:

(a) the angle over which the web of material is passed around thedirection-changing guide means is greater than 140°;

(b) the air pressure in the interior of the direction-changing guidemeans is at least three times the pressure in the air cushion; and

(c) for the purposes of measuring the pressure in the air cushion, thereis a pressure measuring means, the output signal of which is utilised toregulate the motor generator.

Passing the web of material around the direction-changing guide meansover a substantial angle has been found to be effective. In addition,the pressure in the interior of the direction-changing guide means mustbe at least three times the pressure in the air cushion. This provides asatisfactory feed of air for the air cushion, even in the event ofpulsation phenomena. The pressure in the air cushion itself, which isproportional to the strip or band tension, is used as a controlparameter for controlling the motor generator.

More particularly, the air cushion pressure signal is converted in aconverter directly into an electrical proportional signal and suppliedto the regulator means. Together with an output line of a referencevalue generator, a comparison circuit is actuated, the output linethereof carrying the regulating parameter for regulating the motorgenerator. This regulating means ensures that unwinding takes place at aconstant speed of rotation. When the mean air pressure in the aircushion changes, the regulating means is altered on the one side and thespeed of rotation of the motor generator is changed. This operationtakes place very gradually as the diameter of the roll being unwounddecreases. In this arrangement, the expensive booster system illustratedin U.S. Pat. No. 2,945,637 and the drive coupling for the winding-onoperation are completely omitted.

The air forming the air cushion is discharged into the ambientatmosphere along the generating lines and at the annular segments at theends. This gives rise to the danger of the film being caused to flutter.In order for the film to be guided in a smooth and undisturbed mannerand in order for the air to issue from the air cushion as uniformly aspossible, a flow discharge wall means in the form of an apertured platemember is arranged on the entry side and/or the exit side of thedirection-changing guide means, substantially parallel to the web offilm. Air is dissipated through the apertures in the plate member fromthe air cushion, thereby ensuring that the flow is smooth and turbulenceis suppressed.

A further aspect of the invention provides that the length of the commontangent of the direction-changing guide means and the next followingdriven roller is less than 150 mm. It has been found desirable for thenext following driven roller to be disposed as closely as possible tothe air cushion direction-changing guide means in order for the plasticsfilm to be taken over by the driven roller at the minimum possibledistance from the guide means.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the invention is described hereinafter with referenceto the accompanying drawing, the single FIGURE of which shows a partlydiagrammatic general view of the unwinding apparatus in accordance withthe invention.

EMBODIMENT

The unwinding apparatus includes a support means 1 for an unwindingshaft 2 with a roll 3 of a plastics film, which is to be unwound. Theunwinding shaft 2 is driven by a motor generator 4. From the roll 3, theplastics film 5 passes directly to a direction-changing guide means 6which has a cylinder wall means 7 in the form of an apertured plate,which extends at least over an angle of 140° . An increased pressure isproduced in the interior 17 of the direction-changing guide means 6 by ablower 8 so that air flows out through the apertures in the aperturedplate member. That air produces an air cushion 20 between thedirection-changing guide means 6 and the plastic film 5. The arrangementis such that the pressure in the interior 17 is from 2.5 to 4 times thepressure of the air cushion. The pressure in the interior 17 must be atleast 3 times the pressure in the air cushion so as to permit uniformfeed of air to the air cushion, which is sufficient for absorbingpulsation effects.

Following the direction-changing guide means, the plastics film 5 istaken over by a driven direction-changing roller 9. A flow dischargewall means 10 in the form of an apertured plate member is arrangedsubstantially parallel to the path of movement of the web of film. Thecontact pressure of the plastics film against the direction-changingguide means and thus the pressure required for the air cushion iscalculated in accordance with the following known formula:

    p=2.δ.s/D

in which δ denotes the specific unwinding tension which is from 2 to 3N/mm², s is the thickness of the film in millimeters and D is thediameter of the cylinder wall means of the direction-changing guidemeans in millimeters. It is found therefore that the pressure falls withincreasing diameter of the direction-changing guide means. This isdesirable as a lower pressure means less turbulence and thus less noisenuisance. The above formula also shows that the pressure is proportionalto the thickness of the film so that the pressure of the air cushion canbe preset in dependence on the thickness of the film.

The air cushion 20 which is produced by the air flowing out of thedirection-changing guide means 6 through the plate member 7 absorbspulsation effects in the web of film in a virtually inertia-less manner.For the mass of the air cushion and the web of film passing thereover isvery low. The air of the air cushion flows away primarily in theperipheral direction. The air component which is discharged in the axialdirection is comparatively small and does not have a detrimental effecton the air cushion. The flow discharge wall member 10 serves todissipate air from the cushion in the region of the discharge flow ofair, thereby preventing the formation of turbulence in the dischargeflow region, thereby giving a uniform discharge flow.

Desirably, the driven roller 9 is disposed as close as possible to thedirection-changing guide means 6 so that the common tangent of thedirection-changing guide means and the next following drive roller isless than 150 mm in length.

The invention permits regulation of the speed of rotation used in theunwinding operation, and thus provides for uniform unwinding. A pressuremeasuring tube 11 is arranged in the region of a discharge flow aperturein the plate member 7 so that the pressure in the air cushion 20 can bemeasured by means of the pressure measuring tube 11. The pressure in theair cushion 20 is converted in a transducer 12 into a current which,after amplification in an amplifier 13, is fed into a comparison circuit14. The other input of the comparison circuit is connected to the outputline of a reference value generator 15. On its output line, thecomparison circuit 14 produces a regulating voltage which is amplifiedin an amplifier 16. The amplified voltage is fed to the motor generator4 and regulates the speed of rotation of the motor generator 4 and thusthe speed of rotation of the unwinding shaft 2 in such a way that thepressure within the air cushion 20 remains at a constant value which ispreset by the reference value generator 15. A constant pressure withinthe air cushion means a constant value in respect of the strip tensionin the unwinding operation. In this way, the speed of rotation used inthe unwinding process can be easily regulated in accordance with theinvention.

I claim:
 1. In an unwinding apparatus for webs of material, inparticular plastic films, having a roll of the web material, thecircumference of which may be out-of-round, supported on an unwindingshaft driven by a motor-generator and a driven direction-changingroller, the improvements comprising:(a) a direction-changing guide meanshaving a foraminous wall defining a hollow interior, the web materialpassing around the guide means through an angle of at least 140°; (b) asource of pressurized air connected to the guide means such thatpressurized air is supplied to the interior of the guide means, aportion of the air passing out through the foraminous wall to form anair cushion between the guide means and the web material, the airpressure in the interior of the guide means being at least three timesthe pressure of the air cushion to prevent the web material from cominginto contact with the guide means during pulsations in the web caused bythe out-of-roundness of the web roll; (c) pressure measuring means tomeasure the air pressure in the air cushion; (d) control means connectedto the pressure measuring means and the motor-generator to regulate themotor-generator as a function of the pressure in the air cushion; and,(e) a discharge flow wall means in the form of an apertured platelocated adjacent the exit side of the direction changing guide meanssubstantially parallel to the web of film for dissipating the aircushion through apertures in the plate.
 2. An unwinding apparatusaccording to claim 1 wherein the driven direction-changing roller (9)directly follows the direction-changing guide means (6).
 3. An unwindingapparatus according to claim 1 wherein the driven direction-changingroller (9) has the web of material passing therearound at least over anangle of 140° or more and has a rubberised circumferential layer.
 4. Anunwinding apparatus according to claim 3 wherein the length of a commontangent between the direction-changing guide means (6) and thedirection-changing roller (9) is less than 140 mm.